Storagespace

``get_quota`` doesn't what it's documentation suggests.

I provided ``get_storage_space`` that provide both *"used"* space and *"total"* space (*"storage space"* is the name used in mega.co.nz's web interface for those numbers).

that pull request is commited "on top" of pull request #24, if pull request #24 is rejected, you need to ``rebase``.